After a borehole is drilled, it is often useful to gain information about the quality and condition of certain areas of the wellbore. One way to obtain this information is through use of the borehole imaging system. The borehole imaging system provides an output signal, which is indicative of the nature of the borehole. The surface is illuminated with acoustic pulses and the acoustic pulse return signal is used in some fashion to obtain an indication of the surface of the surrounding borehole. This procedure is normally carried out in an open-hole condition where the well is filled with drilling fluid. The wall is intended to be at a controlled and specific distance from the transducer, which transmits and then receives the acoustic pulse. For optimum resolution, the acoustic energy is focused at some specific distance from the logging tool.
It is expected that focusing the acoustic energy will provide two advantages. First, the return signal from a focused acoustic pulse generally has a higher amplitude, which improves the signal-to-noise ratio of the measurement. Second, the focused pulse provides the measurements with increased distance sensitivity, which translates into an improved depth of field. Such sensitivity improves the system's response to surface roughness and other rugosity. Both of these anticipated advantages would contribute to improved detection of formation characteristics, boundaries between formation beds, and faults or other voids intersected by the borehole.
One way to focus the acoustic energy is to employ an annular ring transducer such as that described in U.S. Pat. No. 5,044,462 titled “Focused Planar Transducer” and filed Jul. 31, 1990 by inventor V. Maki. However, this and other existing annular ring transducer designs require deeply cut grooves for their operation. Previous fabrication methods cut grooves with a minimum depth of 80% of the piezoelectric material thickness to form annular rings at the surface. Such grooves can be difficult and expensive to cut, and may be expected to reduce yield and reliability.